Final working solution: shadcn date picker with timezone fix

- Implemented shadcn/ui date picker with Czech localization
- Added month/year dropdown navigation for easy date selection
- Fixed critical timezone bug causing "No valid days found" error
  - Changed from toISOString() to local date formatting
  - Dates now correctly sent as 2025-01-01 instead of 2024-12-31
- Calendar auto-closes after date selection
- All features tested and working:
  - Journey calculation with correct date ranges
  - "Vyplnit na web" button visible and functional
  - Excel export working
  - Backend successfully processes January 2025 data

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/app/components/DataPreview.tsx

@@ -23,6 +23,14 @@ export default function DataPreview({ data, loading, formData }: DataPreviewProp
     setFillResult(null)
 
     try {
+      // Format dates as YYYY-MM-DD in local timezone
+      const formatLocalDate = (date: Date) => {
+        const year = date.getFullYear()
+        const month = String(date.getMonth() + 1).padStart(2, '0')
+        const day = String(date.getDate()).padStart(2, '0')
+        return `${year}-${month}-${day}`
+      }
+
       const response = await fetch(`${API_URL}/api/fill/journeybook`, {
         method: 'POST',
         headers: {
@@ -31,8 +39,8 @@ export default function DataPreview({ data, loading, formData }: DataPreviewProp
         body: JSON.stringify({
           username: formData.username,
           password: formData.password,
-          start_date: formData.startDate,
-          end_date: formData.endDate,
+          start_date: formatLocalDate(formData.startDate),
+          end_date: formatLocalDate(formData.endDate),
           start_km: parseInt(formData.startKm),
           end_km: parseInt(formData.endKm),
           vehicle_registration: formData.vehicleRegistration,
@@ -142,7 +150,7 @@ export default function DataPreview({ data, loading, formData }: DataPreviewProp
         </table>
       </div>
 
-      {formData && formData.startDate === '2025-01-01' && (
+      {formData && (
         <div className="mt-4">
           <Button
             onClick={handleFillToWebsite}